Recently, various safety regulations for protecting passengers in automobiles have been strengthened. In addition, as one measure to improve the fuel efficiency of automobiles, improvements in the strength, along with the lightening of members used in automobiles are becoming important considerations.
Therefore, to this end, a hot press forming method is employed in the manufacturing of automobile parts. Regarding the hot press forming method, after a steel sheet is heated to a predetermined temperature (for example, a temperature at which a phase thereof is changed to an austenite phase) to lower strength, and the steel sheet is formed in a mold having a lower temperature (for example, room temperature) than that of the steel sheet, so formation is given. Simultaneously, quenching is carried out by using a temperature difference of both, so strength after forming is secured. Moreover, the hot press forming method has various names such as a hot forming method, a hot stamping method, a hot stamp method, a die quenching method and the like, in addition to a hot pressing method.
As described above, when hot press forming (for example, hot deep drawing) is performed using a mold, while a steel sheet is heated at a two-phase temperature of (an Ac1 transformation point to an Ac3 transformation point) or at a single-phase temperature of an Ac3 transformation point or more to be softened, forming starts. That is, in a state in which a steel sheet in a high temperature state is inserted into a gap between a die and a blank holder, the steel sheet is pushed into a hole of the die by a punch, and the steel sheet is formed to have a shape corresponding to an outer shape of the punch as an outer diameter of the steel sheet is reduced. Then, the punch and the die are cooled while forming is performed. Thus, while heat diffusion from the steel sheet to the mold and the die is performed, a material is further maintained at a bottom dead center of forming, so quenching of the material is performed. By performing such a forming method, it is possible to obtain a formed product with 1500 MPa having good dimensional accuracy. Moreover, forming load may be reduced as compared with a case of forming a part having the same strength class in the cold, so that the capacity of a press machine is able to be reduced.
The hot press forming method is widely used as a forming method not only capable of normally obtaining an ultrahigh strength product with 1000 MPa or more, but also of obtaining excellent dimensions and precision in products. In addition, a basic material thereof corresponds to a heat-treated steel having a composition system in which Ti and Cr are added in common based on a composition system of 22MnB5, i.e. 0.22% of C-1.2% of Mn-maximum 50 ppm of B, specified in EN standard, and is further hot plated with an aluminum alloy including 9 wt % to 10 wt % of Si, 2.0 wt % to 3.5 wt % of iron (Fe) in addition to aluminum.
However, generally, when a steel sheet hot-dip plated with an aluminum alloy is hot press formed, a thickness of a plated layer increases as the plated layer transforms into a large number of intermetallic compound layers. For example, the large number of intermetallic compound layers including Fe—Al-based oxides such as Fe3Al, FeAl, FeAl2, Fe2Al5, FeAl3 and the like are formed in a direction from iron to a surface. These intermetallic compounds are largely divided into a diffusion layer and an alloy layer. Since the diffusion layer is soft and the alloy layer is hard, fracturing may occur at an interface between the diffusion layer and the alloy layer during hot press forming. As a result, a plated layer is delaminated, so the plated layer in the form of fine powder is stacked on a mold. In some cases, the fine powder adheres to the mold. Therefore, there are problems that various causes such as dimensional variations of a formed article, surface dent induction, mold correction, cutting, and the like may be provided and additional costs may be incurred.
Therefore, when an aluminum-plated steel sheet is employed for use in automobiles and the like, there is a continuing need for a method capable of suppressing cracking and delaminating of a plated layer after such hot press forming.
In detail, in Korea Patent Publication No. KR 2010-0082537, disclosed are a hot press formed product capable of suppressing crack propagation by increasing a (Fe3Al+FeAl) alloy phase layer, preventing crack propagation, and relatively reducing a Fe2Al5 layer, providing a cause of cracking, by optimizing conditions of a plating bath when a hot-rolled steel sheet or a cold-rolled steel sheet is manufactured as an aluminum plated steel sheet, and controlling a manufacturing process [a heating temperature and the heating time are controlled to a lower heating temperature and the shorter maintaining time than an aluminum plated steel sheet according to the related art] in an operation of manufacturing a hot press forming product; and a method of manufacturing the same. However, in the technique disclosed in the above-mentioned publication, there are limits that the formed alloy phase (Fe3Al+FeAl) lacks crack prevention capability and an alloy layer is delaminated in the presence of a cracking alloy phase (Fe2Al5) layer. That is, delamination in the form of fine powder may still occur, causing problems such as mold adhesion, surface dent generation and the like.
Therefore, there is demand for development of a HPF article having excellent press formability, able to overcome the above-mentioned problems.